1. Field of the Invention
The invention relates to an internal combustion engine and a method of controlling the internal combustion engine.
2. Description of the Related Art
In an internal combustion engine having a secondary-air supply passage connected to an engine exhaust passage, and an air pump for supplying secondary air into the engine exhaust passage via the secondary-air supply passage, a pressure sensor for detecting an absolute pressure in the secondary-air supply passage is disposed in the secondary-air supply passage, and fault diagnosis of a secondary-air supply control device is performed based on an output value of the pressure sensor, as disclosed in, for example, Japanese Patent Application Publication No. 2003-83048 (JP-A-2003-83048).
In the internal combustion engine as described above, fault diagnosis is performed based on the magnitude of the pressure in the secondary-air supply passage relative to atmospheric pressure, and it is, therefore, necessary to detect in advance an output value of the pressure sensor representing atmospheric pressure. Also, since the fault diagnosis is carried out immediately after supply of secondary air is started after startup of the engine, the output value of the pressure sensor representing atmospheric pressure must be detected when or before the engine is started. Accordingly, a controller of the engine as described above stores an output value of the pressure sensor detected immediately before startup of the engine, as an output value representing atmospheric pressure.
In the meantime, a battery voltage is applied from a battery to the pressure sensor, and the output value of the pressure sensor decreases as the battery voltage decreases even if the sensor detects the same atmospheric pressure. On the other hand, immediately before startup of the engine, a starter motor or other accessories having a high electric load may be actuated or started. If the starter motor and/or other accessories of a high electric load is/are actuated while the battery is in deteriorated condition, the battery voltage may be reduced. Accordingly, if the output value of the pressure sensor obtained immediately before startup of the engine is simply stored as the output value representing atmospheric pressure while the battery voltage is reduced, as in the above-described engine, there arises a problem that the stored output value of the pressure sensor does not actually represent the atmospheric pressure.